CT Analyzer: Coming to the Rescue. Introduction. Equivalent Circuit. Construction. Classification of a CT. Tony Porrelli, OMICRON, UK

Transcription

1 Presentation 09.1 CT Analyzer: Coming to the Rescue Tony Porrelli, OMICRON, UK Introduction The Current Transformer (CT) may exist for many applications but for this paper we can think about them being for Protection and Measuring purposes. Generally speaking, protection CT s usually connect to protection relays and measuring CT s usually connect to meters. Specifications of CTs in the power network need to be considered carefully. Without going into too much depth, usually they are specified in terms of: Ratio Class Burden Knee Point Various Standards Institutes set methods for manufacturing and testing but more importantly how to classify CT s in terms of Accuracy and Burden. The accuracy class helps us to discriminate between a Protection CT and a Measuring CT. Construction Has anybody ever stopped to think What an interesting device the current transformer is. Essentially formed with four components: an Iron Core Insulation Copper Wire Terminations S1 / X1 S2 / X2 Fig. 1 Components of a Current Transformer (CT) This is of course the simplest form but the construction really depends on the voltage which has to be withstood between the Primary and Secondary. As voltage levels exceed approx. 5kV then porcelain will be included in the design and at even higher voltages, oil-immersion is necessary. How can such a simple assembly create such an aura and complexity? Firstly, we should look at the equivalent circuit. Equivalent Circuit When looking beyond its essential components, we reveal a complex electrical network of impedance s, voltages and currents. The current transformer equivalent circuit can be seen below. Fig. 2 CT Equivalent Circuit The resistance of the secondary winding is much higher than the leakage reactance which is usually ignored. The primary is normally considered to be a single conductor (1 Turn) where its resistance and reactance are so small that they too can be ignored. We see from Fig. 2 that the losses contain two components, that being the reactive (I xm ) and resistive (I rm ) losses making up the total error current, I e. The secondary current available for the load is now I s = (N p /N s x I p ) - I e. It shows that the primary current provides the exciting current for the magnetisation of the core and supplies the hysteresis and eddy losses. The quantity of exciting current is dependent upon the core material and the amount of flux to provide the burden requirements. The current that remains is transformed by the turns ratio to provide the secondary output. The model is crucial to modern testing devices and has been explained in many texts. The secondary load of a CT is termed the Burden and is expressed is volt-amperes (VA).The burden consists of connected instruments and associated leads and connections. Classification of a CT As the CT secondary current should represent the proportion of the primary load current, then

2 Presentation 09.2 accuracy of transformation to the turns ratio is required at rated burden depending on the classification of the CT. Therefore we see that due to the excitation current circulating in the magnetising branch of the circuit, it is not transferred to the secondary side output, the CT output will have deviations to what is expected. This ratio error is the percentage difference between the secondary current multiplied by the transformation ratio expressed as %. An error in phase also exists between the primary and secondary currents owing to the primary having to provide the exciting current. The phase angle being measured in minutes. The phase error is considered positive when the secondary current leads the primary. The simple CT can take on two guises: It can be the sleeping giant of the protection system, ready to roar when poked with a big stick. But also the pedigree workhorse, thanklessly operating day by day to provide the measurement system with accurate information. Measuring CTs must maintain accuracy over a limited range of primary current, typical between 5% and 120% of full load rating. These usually connect to instruments and meters with the advantage that they will saturate early to protect the instruments. Protection CTs however, must maintain accuracy over a wide range of primary current which can be up to 20 or 30 times rated full load value. Normally being connected to protection relays. In the past, CTs were designed to the British Standard BS3938 Specification which was superseded by IEC Manufacturers of CTs can supply to any country and will use the recommended standards that are relevant to that particular region. In many areas, such as Europe, the IEC standards are relevant but exist in many editions. IEC Instrument transformers: Current transformers IEC Combined transformers IEC Requirements for protective current transformers for transient performance IEC that supersedes the IEC60044 range but I will mention this later. Australia AS 1675 Current transformers - Measurement and protection One of the many differences between the IEC and the ANSI/IEEE standards is that the IEC allow secondary current definitions of 0.5, 1, 2, or 5A where the ANSI/IEEE standard defines 5A. CT Testing No matter which standard is used as the basis for design, there are many similarities in the tests that need to be conducted by a manufacturer. Some of basic tests include: Turns ratio by Primary injection Current Ratio by Primary injection, at rated burden when required Secondary winding resistance referenced to 75ºC Magnetisation curve measurement Knee point verification where applicable Accuracy Limit Factor verification for protection CT s such as class 5P & 10P Instrument Security Factor (FS) verification for measuring CT s if required ANSI/IEEE assessment to the Ratio Correction Factor and Phase Angle Parallelogram ANSI/IEEE testing at 0.5 or 0.9 Power Factors Polarity In the past, limited by advances in technology, the traditional measurement system for testing the ratio and burden of a CT has been commonly referred to as Primary Injection, where by various levels of Primary current are passed thought the CT and the secondary output is measured with full load and with additional load conditions according to the given Standard definitions i.e. according to IEC for 25% to 100% rated burden where applicable. The following photo shows how it was carried out in the past and it can be time consuming to set up when you consider a source, a measurement bridge, reference standard, and burden at the correct power factor. Not forgetting the pace of a human operator. For countries such as USA, Canada and Australia the following standards are relevant. ANSI/IEEE Std C : IEEE Standard requirements for Instrument transformers ANSI/IEEE Std C : IEEE Standard for High-Accuracy Instrument Transformers Canada CAN3-C13-M83: Instrument transformers

3 Presentation 09.3 designer would try to use a high flux density as this may lead to less core material and reduced costs. Continuing from this the CT may also require secondary winding resistance measured, and if it is a wound primary CT, possibly even the primary winding resistance. Typically, an old measurement bridge could have been used with great accuracy such as that shown in Fig 5. Fig. 3 Testing bay from the old past Another important measurement is that of the excitation curve, or magnetising curve as it is also known. Among other things, this is used to check the knee point which is defined by the IEC as a 10% increase in voltage giving rise to a 50% increase in current. ANSI/IEEE defines the knee point as the intersection of the excitation current curve with a 45º tangent line respectively or alternatively with a 30 tangent line for gapped core CTs. More test equipment is required that consists of a variac source, step up transformer, an ammeter and a voltmeter. A graph is produced based on the test voltage values and measured currents. This is also very time consuming. The typical circuit is shown below. V(V) Vkp Knee Point A Protection CT S1 S1/X1 Fig. 5 Typical resistance bridge Like all CT s, polarity is an important parameter to verify when connected to polarity-sensitive meters, control and protection relays. A traditional polarity checker can be seen below. It looks to me like something my grandfather would make on a Saturday morning from an old wardrobe. 10% 50% V Saturated Region S2/X2 Linear Region Measurement CT Ankle Point Region I (A) Fig. 4 Conventional mag curve testing circuit & typical measurement curve according to IEC In general, a protection CT would operate in the working range from the Ankle Point to the Knee Point region or above. The measurement CT may work around the Ankle Point region but in some cases can also work just under the knee point. The Fig. 6 Typical polarity checker For a CT manufacturer, these industrial devices may be accessible and permanently installed to assist with testing. But for the commissioning and maintenance engineer, this can become a huge problem. Especially when it comes to primary injection at rated burdens.

4 Presentation 09.4 It also becomes extremely difficult for the on-site engineer to test and verify to any International Standards such as the ANSI/IEEE or IEC. What is a PASS and what is a FAIL? Shouts the exhausted engineer. CT Analyzer to the rescue Firstly, the CT Analyzer (CTA) can conduct the majority of tests required by international standards and perform verification as to whether the CT will pass or fail. CT Analyzer: Rescue 1 All the industrial equipment examples we have previously seen are not really considered to be portable. The transportation requirements would be huge. The ideal solution would be for manufacturers and on-site engineers to be more streamlined and efficient taking only one device to cover most of the important electrical tests required. In addition to testing efficiency, test reports are generated automatically saving even more time. Weighing in at 8kg, the OMICRON CT Analyzer is the revolutionary lightweight test unit capable of testing Ratio, Burden, Resistance, Magnetisation Curve and much more taking only about 60 seconds to complete all tests, even referencing the resistance to 75ºC and providing the Remanence. It is extremely simple to use and in the event that there is no information about the CT under test, the unit can use the measured electrical data to provide an educational guess as to what classification the CT can meet. Ratio s up to 10000/1 and more can be tested. Knee point voltages up to 30kV can be checked. CT Analyzer: Rescue 2 Recently, however, the IEC made a decision to consolidate the many editions of the IEC60044 documents and now the new editions exist as follows: IEC Instrument transformers: General requirements IEC Additional requirements for current transformers This can impact manufactures and on-site engineers as they may now come up against a choice of many different standards. The CTA comes to the rescue in this instance as OMICRON has already implemented the automatic assessment to the IEC into its software. This gives the user easy access to verify against a multitude of international standards. The ANSI/IEEE Standard C is also under rework being launched sometime in the future. Again, the CT Analyzer will be ready to validate CTs to the new specifications. CT Analyzer: Rescue 3 Planning the requirements of a substation is very complex. It is no surprise that specifications constantly change, often very quickly after the initial design of an asset has been accepted and manufactured. For High Voltage Current Transformers (HV CT s) where there are expensive bushings and copper work, this can become challenging in terms of design and more importantly, cost. OMICRON s CT Analyzer helps determine the CT classification and can simulate various loads after a design change, in timely manner. Manufacturing companies put great effort into perfecting the manufacturing process in order to create instruments of high accuracy and quality. This is particularly true for the production of HV CTs where manufacturers have a real challenge to ensure the accuracy and quality of such high value equipment. Where critical specification changes are necessary, manufactures need to provide quick solutions, preferably limiting the need to re-design the product. Changed Specifications This scenario was encountered by a HV CT manufacturer, whereby their end customer had specified a number of multi-tapped high accuracy Class 0.2s units to be supplied. However, after the design had been accepted, manufactured and tested, the customer then requested a change to the specification. The challenge for the manufacturer was how to address the customers' needs with minimal impact

5 Presentation 09.5 on the design and cost. A further complication was the requirement for HV CT s to undergo approval testing at an overseas independent test laboratory. The HV CTs that already been dispatched overseas for testing were successfully approved to the original nameplate specification defined. options. Importantly, this device allows the user to play around with various test specifications. With this equipment, it is easy to make connections and to apply different test loads to the HV CT under test to evaluate its performance. The CT Analyzer helped to determine the load that can be applied to the HV CT to retain the classification of 0.2s. The manufacturer was thus able to return the HV CT to the independent overseas laboratory for approval, knowing that the equipment would successfully pass. The investigation and tests completed with the CT Analyzer took as little as 30 minutes. This would be impossible using traditional testing equipment, particularly given the location of the HV CTs and the time constraints. Literature [1] Connelly F.C.: Transformers: Their principles and design for light electrical engineers, Printed in UK, First Published 1950 [2] Brian D. Jenkins: Introduction to Instrument Transformers, Printed in UK, First Published 1967 [3] Jaun M. Gers and Edward J. Holmes: IET, Protection of Electricity Distributoin Networks, 2 nd Edition [4] IEC :2003 Standard Fig. 7 Example of the HVCT under test Due to size and weight, expensive shipping costs were incurred. The approved CT s were returned to the manufacturer and at this point, the end customer requested to re-define the CT tap, the VA rating and retain the Class 0.2s classification. This change meant that the units, following the respecification, would need to be returned to the overseas test laboratory for re-approval to the new specification. To avoid unnecessary shipment costs, and to avoid delay, the manufacturer wanted to be confident that the new specification would be met be the CT before returning it to the laboratory. Conducting traditional primary injection testing of this type of HV CT requires a powerful source and considerable time and effort to complete the required testing efficiently. However, the site where the HV CTs were stored was not the same site as the factory test site. About the Author Tony Porrelli attained his BEng (Hons) back in 1993 and many many years later left is home of Scotland and moved to England on January 1 st 2008 to begin his employment with OMICRON. Before joining OMICRON he was a current transformer design engineer based in Scotland. Now he has the responsibility of being the Regional Application Specialist for Instrument Transformers covering Europe and Africa. This is in addition to his role of providing technical and application support to all OMIRCON products covering the United Kingdom and Ireland Verification in 30 minutes OMICRON was contacted by the manufacturer who wanted assistance with this testing. It was recommended to use the CT Analyzer, a fast, portable, and easy to use test device which provides the user with a broad range of testing